The present invention relates to cellular communication systems, and more particularly to the handover of calls between cellular communication systems that support different security contexts.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often generically referred to as “base stations”, even when particular communication standards setting bodies apply different terminology (e.g., “NodeB” in WCDMA, and “eNodeB” in LTE) for the purpose of very precisely pointing out the distinctive capabilities and architectures of their version of the base station. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. Handovers are controlled by a system-defined cell reselection mechanism. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
As new communication systems come into existence, they bring with them new features, capabilities, and ways of handling calls. The mobile communication equipment (hereinafter referred to as “User Equipment”, or “UE”) must operate in a way that is compatible with the system with which it is expected to communicate. In order to provide the most flexibility with respect to usage, UEs are often designed to be compatible with more than one system. In one respect, this enables a user to continue using the UE as it is carried from a geographical area covered by one type of communication system into another area, served by a different type of communication system.
Having multi-mode capability is also useful because newer systems are often rolled out piecemeal, so that even if a user stays within the geographical confines of one operator's system, the UE may find itself at times served by older equipment, and at other times served by newer equipment. This situation is illustrated in FIG. 2, which depicts a portion of a cellular communication system in which a UE 201 is presently being served within a first cell 203 that is supported by equipment 205 that conforms to an older communications standard (e.g., one of the 2G—e.g., GERAN—or 3G—e.g., UTRAN—standards). In this example, the UE 201 is in the vicinity of a second (neighboring) cell 207 that is supported by equipment 209 that conforms to a newer communication standard (e.g., a 4G specification, such as E-UTRAN which is also known as “Long Term Evolution” or “LTE”). If the user is engaged in a call at the time that a handover should be performed from the older equipment 205 to the newer equipment 209 it would be desirable to be able to handover the call in a graceful way that minimizes the call's disruption.
However, since older communication systems are not designed with the knowledge of what information will be required to support a handover to newer equipment, designers have been faced with the problem of how best to enable such handovers to take place (i.e., how to supply the new equipment with information that puts it in the best position to pick up support for the ongoing call that is presently served by older equipment). Solutions to this problem, involving methods, apparatuses, and/or software are therefore desired.